In System Programming (ISP) for ATMEL chips

If you didn’t guess it from the title, ISP is a way to serially program your microcontroller, while it resides in its place, in other words, without removing the chip from your board.

Whether you’re just starting in the ATMEL microcontrollers, or you’re familiar with it, ISP (In System Programming) will provide you a simple and affordable home made solution to program and debug your microcontroller based project.

Sometimes, ISP can become very useful, when adjusting some delays, frequencies or any other values that you would intend to find by trial and error.. a process that would otherwise take too much time.

How does ISP works?

Normally, the flash memory of an ATMEL microcontroller is programmed using a parallel interface, which consists of sending the data byte by byte (using eight independent lines for the data, and another bunch of lines for the address, the control word and clock input). On the other hand ISP is performed using only four lines, and literally, data is transferred through two lines only, as in a I2C interface, where data is shifted in bit by bit though.

MOSI (data input) line, with a clock cycle between each bit and the next (on the SCK (clock input) line). MISO (data output) line is used for reading and for code verification, it is only used to output the code from the FLASH memory of the microcontroller. The RST (used to activate the serial Programming) pin, which is normally used to reset the device, is also used to enable the three pins (MOSI, MISO and SCK) to be used for ISP simply by setting RST to HIGH (5V), otherwise if RST is low (0V), your program start running and those three pins, are used normally as P1.5, P1.6 and P1.7.

Here is ISP Programming sequence as described in ATMEL datasheets:

“The Code memory array can be programmed using the serial ISP interface while RST is pulled to VCC. The serial interface consists of pins SCK, MOSI (input) and MISO (output). After RST is set high, the Programming Enable instruction needs to be executed first before other operations can be executed. Before a reprogramming sequence can occur, a Chip Erase operation is required. The Chip Erase operation turns the content of every memory location in the Code array into FFH. Either an external system clock can be supplied at pin XTAL1 or a crystal needs to be connected across pins XTAL1 and XTAL2. The maximum serial clock (SCK) frequency should be less than 1/16 of the crystal frequency. With a 33 MHz oscillator clock, the maximum SCK frequency is 2 MHz.“

This is as deep as i got in the ISP process, as i am using a ready made software that will handle the transfer protocol. Some more detailed information about the ISP functioning can be found at www.8052.com and at www.avrfreaks.net. But this is all you need to know to build and use this extra simple programming device. Now that you know some theory about the ISP, you should be ready to build the hardware interface. As you will soon discover, it maybe the simplest circuit that will find in this web site!

The circuit

This circuit is a modification from an original deign of Jerry Meng, and it is compatible with the software/cable proposed at

The schematic above indicates a 74LS08 IC, and I did build the programmer with a 74LS08 IC. However, using a 74HC08, or even better a 74HCT08 IC would be much safer, will give you a better performance and allow you to use a longer cable.

The circuit almost talks for itself, the only part that may need some explanation, is the four AND gates. Connecting both two inputs of the AND gates as shown makes it act like a buffer, to protect the parallel port (Shortly, a buffer is a device that will isolate two circuits).

Then, the pins P1,P2,P3,P4 have to be connected respectively to P1.5, P1.6, P1.7 and RST in the microcontroller.

Finally, J1 is the connection to the computer parallel port.

The PCB and the housing

I used an old parallel printer cable, and the whole circuit is mounted inside the the connector plastic box.

A glance at this pictures may be enough to understand how the PCB is mounted and welded to the parallel port connector. To achieve this, some precision have to be taken in account when producing the PCB.

Note than PINs 14 to 25 of the parallel port are on the Components side of the board, and thus cannot be welded.. but luckily, from all those pins we only need the Ground (0V) (pins 18 to 25), which were all connected to the board, simply through a single jumper wire. (you can notice it, it’s the orange wire in the shown picture at the left)

Below is another overall view of the device before being encapsulated. Also notice in the microcontroller end of the cable, which is simple the female connector of standard pin header. I wont give more details about this part, i’ll leave it to your imaginations! The important thing is to solder the cable to a secure connector instead of leaving the wires free to touch each others, which could cause some damage the the buffer circuit.

Here is a shot of the device in action. the picture may not be as clear as in reality, but i found the High Brightness LED to be cooler than what i’ve imagined..

Here is a shot of some other ISP programmers I made for some of my friends.

Connecting the programmer to the micro controller

Many visitors were confused on how to connect this programmer to the microcontroller. Well, in general, ISP is made to program the microcontroller while it resides in its place, so all the standard connection for the microcontroller to run properly are to be made.

As Mr Sarma – a regular visitor, reviewer and a friend – suggested, here are some examples showing how to connect the programmer to different types of microcontrollers:

Connecting the programmer to an AT89S52

Connecting the programmer to an ATMEGA16L

Note that with the ATMEGA AVR family, You don’t need to add a crystal resonator, as those chips contains an internal resonator, making it ready to use simply by connecting the 5V and GND supply rails.

The programmer software

This is the piece of software that will take the HEX file generated by whatever compiler you are using, and send it – with respect to the very specific ISP transfer protocol – to the microcontroller.

The software I am proposing was designed to be compatible with a multitude of ISP programming cables. in most of cases, all the difference is the pins used on the parallel port to perform the data transfer. The only thing you need to do is to go through ‘Setup’, and chose the ‘AEC ISP’ cable pinout. once chosen, you will notice that it matches the circuit on this page.

If you have more than one parallel port (never seen this a lot..) chose the one to which the programmer is connected.

User contributions

Khizer Naeem

Khizer Naeem, a student from International Islamic University of Islamabad, shares his experience in building the ISP programmer found on this page. His various trials and hard work paid out when he figured out a way of enhancing this ISP programmer. I thank him for sharing his work, quoted is Khizer Naeem little story: “I was working on the isp programmer for more than three weeks after i get it working. I have learned a lot of things and would like to bring in your kind notice.

I have made five attempts of making the isp programmer given on your site, one on bread board two on vero boards, and two on single sided PCBs. All of these were having the same problem. They didn’t worked the most of the time,like they worked 1:100 attempts to program the microcontroller. Some time the signature was faulty, sometimes good. Some time the Writing works and the Verifying fails and most of the times writing fails. I tried shortening the cable and even i made a isp programmer in which the ZIF socket was mounted on the parallel port side board (no cable LOL)

PCB Khizer Naeem

I then tried making the Asim khan’s ISP programmer, its result were worst, “No response from the microcontroller”.I then build the Asim khan’s SPI stand alone programmer the problem was same, Working some times and failing most of the time!

I was about to give up and even arranged the few bucks to purchase a branded universal programmer, when i struck up with an idea that brought me back to life in the programmer business.

I usually used the DB-25 Right angle PCB mount socket. I always common grounded the 18~25 pin of the parallel port with my circuit. This is not enough! You must also ground your circuit with the aluminum cover of the socket which is attached internally to the foil of the parallel cable. I am talking about the cover that surrounds the pins of the male DB-25 socket. Doing this trick, I came back to life and all of my programmers that won’t work previously started working (Now i am having a dozen of working programmer LOLs)“

Eddie Wandy

Eddie Wandy, a electronics hobbyist among us at Ikalogic.com build his programmer based on the data provided on this page, along with his creative touch…

He used USB power supply to power the programmer, and used high quality network cable to connect the programmer to a PC’s prallel port. He used a cable as long as 1.3 m. “This cable is rated to 550Mhz to provide additional performance and bandwidth over and above the basic requirements for data communication.. this RJ45 UTP Tech. cable cant be cut easily like others communication cable, its more heavy duty because the copper inside covered by PVC, not rubber..” says Eddie Wandy.

His configuration is the following:

12 MHz Crystal (oscillator)

22n F ceramic cap (decoupling)

220 µf 16vdc (to refilter and stablelize power supply came from USB port)

outcoming 5Vdc power supply from USB port

I change regular LPT cable with RJ45 network cable (1.3 meters)

If the programmer doesn’t work

Now if your programmer doesn’t work, don’t panic and check the following:

Make sure the cable is no longer than 50 cm (though i made it work with a 1meter cable..) long cables tend to increase noise interference especially with TTL devices.

Try more than one device, I had two AT89S52 which which had their ISP port damaged somehow, as they could be programmed through the parallel programmer and functioned correctly, but wouldn’t let me to program them through ISP.

If you can read a hex file, which was written by any other programmer , but you CANNOT Write another HEX file using the ISP programmer, this is – without a doubt – caused by a damaged chip.

Post your question in the form below, if the solutions above didn’t work.

good evening, sir
I’m having the same problem that Rachel, help me?
I’m trying to write an AT89S8253 school technique works well on the parallel port for connecting a computer, and using a 74LS14 in the cable. I liked to know if is possible to use a converter USB / parallel port to record in a noteboock AT89S8253. thanks for attention.
Att